fix: put SDL_WndProc behind _WIN32 ifdef (Kenix3#423)

Author: briaguya-ai

src/graphic/Fast3D/gfx_sdl2.cpp

@@ -62,7 +62,10 @@ static void (*on_fullscreen_changed_callback)(bool is_now_fullscreen);
 static bool (*on_key_down_callback)(int scancode);
 static bool (*on_key_up_callback)(int scancode);
 static void (*on_all_keys_up_callback)(void);
+
+#ifdef _WIN32
 LONG_PTR SDL_WndProc;
+#endif
 
 const SDL_Scancode lus_to_sdl_table[] = {
     SDL_SCANCODE_UNKNOWN,